Conventionally, a soft start circuit is used for a switching power source in order to prevent ICs or the like acting as a load for a switching transistor or the switching power source from being destroyed or malfunctioning by a rush current provided for charging an output capacitor when the switching power source is turned on.
Now, FIG. 14 will be referred to. FIG. 14 shows an exemplary circuit configuration of a switching power source 1000 of a voltage buck mode (having a relationship of power source voltage (Vcc) >output voltage (Vo); also referred to as a “Buck Mode”) for outputting a positive voltage. The conventional switching power source 1000 shown in FIG. 14 includes a soft start circuit 1001, a triangular wave generation circuit 1002, an error amplifier 1003, a comparator 1004, a drive unit 1005, a switching transistor Tr, a diode D, a coil L, a capacitor C, and resistors R1 and R2. An output voltage from the soft start circuit 1001 is input as a reference voltage Vref for the error amplifier 1003. The error amplifier 1003 is supplied with the output voltage Vref from the soft start circuit 1001 and a voltage obtained by dividing the output voltage Vo by the resistors R1 and R2. A source or a drain of the switching transistor Tr is supplied with Vcc. The soft start circuit 1001 includes a clamp circuit (not shown) for controlling the output voltage therefrom such that the output voltage does not exceed Vref.
When the switching power source 1000 operates normally, the error amplifier 1003 is in a virtual short state in which the two inputs thereof (non-inverted input and inverted input) have substantially the same potential. When the input impedance of the error amplifier 1003 is sufficiently high, the following mathematical expression (1) is fulfilled.
Mathematical Expression 1Vo=(1+R1/R2)Vref  (1)
As is clear from mathematical expression (1), the output voltage Vo is in proportion to the reference voltage Vref. Therefore, by increasing the reference voltage Vref until reaching a certain level after the power is turned on, the output Vo in proportion to the reference voltage Vref also gradually increases.
FIG. 15 is an operation waveform diagram of the conventional switching power source 1000. As understood from FIG. 15, the power source voltage Vcc is turned on at time t1 and the reference voltage Vref is gradually increased until reaching a certain level, so that the output voltage Vo in proportion to the reference voltage Vref also gradually increases. This time period corresponds to a soft start period (t2-t1).
Now, FIG. 16 will be referred to. FIG. 16 shows an exemplary circuit configuration of another conventionally used switching power source 1100 of the voltage buck mode for outputting a positive voltage. The conventional switching power source 1100 shown in FIG. 16 includes a soft start circuit 1101, a triangular wave generation circuit 1102, an error amplifier 1103, a comparator 1104, a drive unit 1105, a switching transistor Tr, a diode D, a coil L, a capacitor C, and resistors R1 and R2. The error amplifier 1103 is a three-input error amplifier, and is supplied with an output voltage from the soft start circuit 1101, a reference voltage Vref, and a voltage obtained by dividing an output voltage Vo by the resistors R1 and R2. A source or a drain of the switching transistor Tr is supplied with Vcc.
The switching power source 1100 acts in substantially the same manner as the switching power source 1000, except that whereas the soft start circuit 1001 in the switching power source 1000 requires a clamp circuit, the soft start circuit 1101 in the switching power source 1100 does not require a clamp circuit.
FIG. 17 is an operation waveform diagram of the soft start circuit 1101. Immediately after the power is turned on at time t1, the output voltage of the soft start circuit 1101 changes in proportion to time because the voltage of the capacitor in the soft start circuit 1101 is 0V. The time period until the output from the soft start circuit 1101 reaches the reference voltage Vref (t2-t1) corresponds to a soft start period.
FIG. 18 is an operation waveform diagram of the conventional switching power source 1100. As understood from FIG. 18, the power source voltage Vcc is turned on at time t1 and the reference voltage Vref is gradually increased until reaching a certain level, so that the output voltage Vo in proportion to the reference voltage Vref also gradually increases. This time period corresponds to the soft start period (t2-t1). The output voltage Vo fulfills mathematical expression (1) shown above.
Another conventionally used switching power source adopts a soft start system by which the output voltage is gradually increased by gradually extending the time period in which the switching transistor is in an ON state (On Duty) from the start of the circuit. Such a soft start system is referred to as the “DTC (Dead Time Control) system”. Patent document 1 mentioned below discloses such DTC system soft start.
FIG. 19 will be referred to. FIG. 19 shows an exemplary circuit configuration of a conventional switching power source 1200 of the voltage buck mode for outputting a positive voltage. The switching power source 1200 adopts the DTC system soft start. The conventional switching power source 1200 shown in FIG. 19 includes a soft start circuit 1201, a triangular wave generation circuit 1202, an error amplifier 1203, a comparator 1204, a drive unit 1205, a switching transistor Tr, a diode D, a coil L, a capacitor C, and resistors R1 and R2. The error amplifier 1203 is a two-input error amplifier, and is supplied with a voltage obtained by dividing an output voltage Vo by the resistors R1 and R2 and a reference voltage Vref. The comparator 1204 is a three-input comparator, and is supplied with an output from the error amplifier 1203, an output from the soft start circuit 1201, and an output from the triangular wave generation circuit 1202. A source of the switching transistor Tr is supplied with Vcc.
FIG. 20 is an operation waveform diagram of the switching power source 1200. As shown in FIG. 20, the output voltage Vo from the switching power source is gradually increased by gradually increasing the voltage at an output terminal of the comparator 1204 for a certain time period. The output voltage Vo fulfills mathematical expression (1) shown above. The threshold level shown in FIG. 20 is determined by the input voltage Vcc and the output voltage Vo.
The switching power source 1200 has the following drawback. Until the soft start is completed, the feedback function is only operative to the comparator 1204. Immediately before the soft start is completed to allow the feedback function to be normally operative and thus the output voltage Vo becomes stable, an over voltage is generated temporarily by the overshoot of the output voltage Vo.
Patent document 1: Japanese Laid-Open Patent Publication No.